Dyed receiving elements which comprise a heterocyclic hydrazono dye

ABSTRACT

Dye donor element for use according to thermal dye sublimation transfer comprising a support having thereon a dye layer comprising at least one heterocyclic hydrazono dye, wherein said dye corresponds to the general formula: 
     
         A═N-N═Het 
    
     wherein A is a carbocyclic or heterocyclic coupler group or a cyclic reactive methylene system, which may carry at least one substituent, and Het is a heterocyclic nucleus, which may carry at least one substituent.

CROSS-REFERENCE TO RELATED U.S. APPLICATIONS

This application is a divisional of application Ser. No. 08/169,335, filed on Dec. 17, 1993, now U.S. Pat. No. 5,468,258.

FIELD OF THE INVENTION

The present invention relates to novel magenta and cyan heterocyclic hydrazono dyes (also called azino dyes), to dye-donor elements and black-coloured dye-donor elements comprising such dyes for use according to thermal dye transfer methods.

BACKGROUND OF THE INVENTION

Thermal dye sublimation transfer also called thermal dye diffusion transfer is a recording method in which a dye-donor element provided with a dye layer containing sublimable dyes having heat transferability is brought into contact with a receiver sheet and selectively, in accordance with a pattern information signal, is heated by means of a thermal printing head provided with a plurality of juxtaposed heat-generating resistors, so that dye is transferred from the selectively heated regions of the dye-donor element to the receiver sheet and forms a pattern thereon, the shape and density of which is in accordance with the pattern and intensity of heat applied to the dye-donor element.

A dye-donor element for use according to thermal dye sublimation transfer usually comprises a very thin support e.g. a polyester support, one side of which has been covered with a dye layer comprising the printing dyes. Usually, an adhesive or subbing layer is provided between the support and the dye layer. Normally, the opposite side of the support is covered with a slipping layer that provides a lubricated surface against which the thermal printing head can pass without suffering abrasion. An adhesive layer may be provided between the support and the slipping layer.

A dye-image receiving element for use according to thermal dye sublimation transfer usually comprises a support, e.g. paper or a transparant film coated with a dye-image receiving layer, into which the dye can diffuse more readily. An adhesive layer may be provided between the support and the receiving layer. A releasing agent may be contained in the receiving layer or in a separate layer on top of said receiving layer to improve the releasability of the receiving element from the donor element after the dye transfer has been effected.

The dye layer can be a monochromic dye layer or it may comprise sequential repeating areas of differently coloured dyes e.g. dyes having a cyan, magenta, yellow, and optionally black-colour hue. When a dye-donor element containing three or more primary colour dye areas is used, a multicolour image can be obtained by sequentially performing the dye transfer process steps for each colour area.

Black-coloured images can be obtained by thermal dye sublimation transfer printing either by sequentially performing the dye transfer process steps for the three primary colours cyan, magenta, and yellow by means of a dye-donor element comprising sequential repeating areas of cyan, magenta, and yellow coloured dyes or by performing only one transfer step by means of a dye-donor element having a black-coloured dye layer containing a fixture of yellow, magenta, and cyan coloured image dyes. The latter of these two methods is preferred because of i.a. the ease of manufacturing the donor element containing only one dye area, less time-consuming recording with only one transfer step, and avoiding the problem of transfer in register of the respective dyes in the respective dye areas. Mixtures of yellow, magenta, and cyan dyes for forming a black-coloured dye layer of such a black-coloured dye-donor element have been described in e.g. European patent application N° 92202157.1, EP 453,020, U.S. Pat. No. 4,816,435, and JP 01/136,787.

An important application of the recording of monochromic black images by thermal dye sublimation transfer is the recording on transparant film receiver of hard copies of medical diagnostic images formed by e.g. ultrasound techniques. Such a hard copy is considered to be an ecologically more acceptable and more convenient substitute for the black-and-white silver hard copy formed by development of conventional photographic silver halide film materials where processing solutions comprising silver salt residues have to be treated carefully before disposal.

In order to be a really valid substitute for conventional photographic silver halide materials the black-coloured fixture of organic dyes used in thermal dye sublimation transfer printing should behave optically as black silver.

In the medical world physicians and radiologists evaluate X-ray photographs or other images on a light box or negatoscope. These light boxes contain fluorescent lamps as light source. The spectral emission of fluorescent lamps depends on the phosphors used in the fluorescent lamp, said phosphors having peak emissions. As a result, fluorescent lamps do not show a continuous emission spectrum. Furthermore, there is no standardization in the type of fluorescent lamp used in said negatoscopes.

There is not so much a problem when viewing classical medical images composed of silver metal on the light boxes, since the spectral absorption characteristics of silver are constant over the whole visible spectrum. The hue of the silver image does not change whatever the spectral properties of the light source are, by which the image is being viewed.

However, when the black image is composed of coloured dyes, problems of hue changes may arise, because the spectral absorption characteristics of organic dyes are not constant over the whole range of the visible spectrum. A black-coloured dye mixture having a neutral look when viewed with one light source will not have a neutral look when viewed with a spectrally different light source. This phenomenon of hue change of an image when viewed with a different light source is highly unwanted, especially when medical diagnostic images have to be evaluated.

The characteristics of colour dyes that can be used for composing black-coloured dye mixtures have been further described in EP-A 453020.

Many of the dyes proposed so far for use in thermal dye sublimation transfer are not sufficient in performance since they yield inadequate transfer densities at reasonable coating coverages, or because they have inadequate spectral characteristics for substractive colour systems, or because they have a poor light-fastness.

As mentioned before, for forming a black record by thermal dye sublimation transfer the transfer currently has been performed by means of a dye-donor element having a black-coloured layer containing a mixture of yellow, magenta, and cyan coloured dyes.

However, the conventional materials are insufficient in performance in that the density of the transferred black image obtained therefrom is too low, especially When transfer has been performed onto a transparent dye-image receiving element.

To fulfil the above described requirements for black-coloured dye mixtures, it would be an advantage that the colour dyes have a high molar extinction coefficient at the absorption maximum combined with high side absorptions. As a consequence, fewer dyes and/or smaller amounts of dyes would be needed to reach higher black density values, which would also result in less overloading of the polymer matrix of the dye donor layer. SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide novel magenta and cyan heterocyclic hydrazono dyes having a high molar extincton coefficient.

It is another object of the present invention to provide novel magenta and cyan heterocyclic hydrazono dyes having a high molar extinction coefficient combined with high side absorptions.

It is another object of the present invention to provide dye donor elements, in particular magenta or cyan dye donor elements, that yield transferred dye images having high densities.

It is a further object of the present invention to provide black dye donor elements that yield transferred black images having high densities.

Further objects will become apparent from the description hereinafter.

In accordance with the present invention a dye donor element for use according to thermal dye sublimation transfer is provided, said dye donor element comprising a support having thereon a dye layer comprising at least one heterocyclic hydrazono dye, wherein said at least one heterocyclic hydrazono dye corresponds to the following general formula (I): ##STR1## wherein: z represents the atoms necessary to complete a heterocyclic ring system, a substituted heterocyclic ring system including a heterocyclic ring system carrying a fused-on aliphatic or aromatic ring system,

Q₁, and Q₂ (same or different) represents hydrogen or a substituent e.g. an alicyclic, aromatic, or heterocyclic ring system including such ring system in substituted form,

R₂ represents an alkyl group, a substituted alkyl group, a cycloalkyl group, a substituted cycloalkyl group, an aryl group, or a substituted aryl group,

m is 0 or 1 and

Y represents the atoms necessary to complete a heterocyclic coupler system or substituted heterocyclic coupler system.

The present invention also provides novel dyes corresponding to general formula I.

The present invention further provides a dyed receiving element comprising a dye in image-wise distribution, formed by thermal dye sublimation transfer using a dye-donor element according to the present invention.

The present invention also provides a method of forming an image by image-wise heating a dye-donor element comprising a support having thereon a dye layer comprising a binder and at least one dye corresponding to any of the above general formula I, and causing transfer of the image-wise heated dye to a receiver sheet.

DETAILED DESCRIPTION OF THE INVENTION

The dyes of the invention corresponding to the general formula (I) can be prepared according to methods known to those skilled in the art of organic synthesis, e.g. by oxidative coupling of appropriate heterocyclic hydrazones, preferably heterocyclic sulfonylhydrazones, with the appropriate couplers in a basic environment. This will become more apparent from the preparation examples hereinafter.

Appropriate couplers have been described in EP 279,467, EP 480,252, EP 454,049, EP 362,808, JA 3016792, JA 02084391, JA 03081192, and JA 02181747.

The synthesis of hydrazono and sulfonylhydrazono compounds has been described in FR 1,444,971, GB 1,301,657, GB 1,286,831, U.S. Pat. No. 3,622,327, U.S. Pat. No. 3,839,035, GB 1,392,433, and U.S. Pat. No. 4,004,926.

Representatives of dyes that can be used according to the present invention are listed in the following Table 1.

                  TABLE 1                                                          ______________________________________                                          ##STR2##                     C1                                                ##STR3##                     C2                                                ##STR4##                     C3                                                ##STR5##                     C4                                                ##STR6##                     C5                                                ##STR7##                     C6                                                ##STR8##                     C7                                                ##STR9##                     C8                                                ##STR10##                    C9                                                ##STR11##                    C10                                               ##STR12##                    C11                                              ______________________________________                                    

The following preparation example 1 illustrates the synthesis of the dyes corresponding to the general formula (I).

Preparation example: synthesis of dye C10

Dye C1 is prepared according to the following reaction scheme 1. Compounds E and F are prepared according to methods known to those skilled in the art of organic synthesis. ##STR13##

8 g (14.8 mmole) of compound E, 3.3 g (16.3 mmole) of compound F, and 3.3 ml of ammonium hydroxide (3 equivalents) are dissolved in 200 ml of ethanol. The solution is refluxed. A solution of 10.3 g of potassium cyanoferrate (III) in 50 ml of water is added dropwise thereto and the resulting solution is stirred for 10 min. After cooling the precipitate is filtered and purified by boiling in 100 ml of tert-butyl methyl ether. Yield: 3.2 g. of dye C1.

The dyes can be used as filter dyes e.g. for silver halide colour photographic materials and also as antihalation dyes. They can be used in inkjet printing, resistive ribbon printing, in inks e.g. for laser applications, in textile, in lacquers, and in paints. They can also be used for transfer printing on fabrics.

According to a preferred embodiment of the present invention the dyes are used in the dye layer of a dye-donor element for thermal dye sublimation transfer.

The dye layer of the dye-donor element is formed preferably by adding the dyes, a polymeric binder medium, and other optional components to a suitable solvent or solvent mixture, dissolving or dispersing by ball-milling these ingredients to form a coating composition that is applied to a support, which may have been provided first with an adhesive or subbing layer, and dried.

The dye layer thus formed has a thickness of about 0.2 to 5.0 μm, preferably 0.4 to 2.0 μm, and the amount ratio of dye to binder ranges from 9:1 to 1:3 by weight, preferably from 2:1 to 1:2 by weight.

The following polymers can be used as polymeric binder: cellulose derivatives, such as ethyl cellulose, hydroxyethyl cellulose, ethylhydroxy cellulose, ethylhydroxyethyl cellulose, hydroxypropyl cellulose, methyl cellulose, cellulose nitrate, cellulose acetate formate, cellulose acetate hydrogen phthalate, cellulose acetate, cellulose acetate propionate, cellulose acetate butyrate, cellulose acetate pentanoate, cellulose acetate benzoate, cellulose triacetate; vinyl-type resins and derivatives, such as polyvinyl alcohol, polyvinyl acetate, polyvinyl butyral, copolyvinyl butyral-vinyl acetal-vinyl alcohol, polyvinyl pyrrolidone, polyvinyl acetoacetal, polyacrylamide; polymers and copolymers derived from acrylates and acrylate derivatives, such as polyacrylic acid, polymethyl methacrylate and styrene-acrylate copolymers; polyester resins; polycarbonates; poly(styrene-co-acrylonitrile); polysulfones; polyphenylene oxide; organosilicones such as poly-siloxanes; epoxy resins and natural resins, such as gum arabic. Preferably, the binder for the dye layer of the present invention comprises cellulose acetate butyrate or poly(styrene-co-acrylonitrile).

The dye-donor element of the present invention can be used for the recording of a coloured image together with primary colour dye-donor elements, comprising respectively a magenta dye or a mixture of magenta dyes, a cyan dye or a mixture of cyan dyes and a yellow dye or a mixture of yellow dyes.

Any dye can be used in such a primary colour dye layer provided it is easily transferable to the dye-image-receiving layer of the receiver sheet by the action of heat.

The dyes of the present invention can be used alone or mixed with one another, or even mixed with other primary colour dyes.

The dyes according to formula (I) are particularly useful for making black and white images using the thermal transfer process. Such black and white images may be composed of 3 primary dyes, i.e. yellow, magenta and cyan, that are transferred in sequence to an image receiving element or said black and white images may be obtained by transferring a black mixture of several dyes. In each case at least one of the constituting dyes will be a dye according to formule (I).

Typical and specific examples of other primary colour dyes for use in thermal dye sublimation transfer have been described in e.g. EP 400,706, EP 209,990, EP 216,483, EP 218,397, EP 227,095, EP 227,096, EP 229,374, EP 235,939, EP 247,737, EP 257,577, EP 257,580, EP 258,856, EP 279,330, EP 279,467, EP 285,665, U.S. Pat. No. 4,743,582, U.S. Pat. No. 4,753,922, U.S. Pat. No. 4,753,923, U.S. Pat. No. 4,757,046, U.S. Pat. No. 4,769,360, U.S. Pat. No. 4,771,035, JP 84/78,894, JP 84/78,895, JP 84/78,896, JP 84/227,490, JP 84/227,948, JP 85/27,594, JP 85/30,391, JP 85/229,787, JP 85/229,789, JP 85/229,790, JP 85/229,791, JP 85/229,792, JP 85/229,793, JP 85/229,795, JP 86/268,493, JP 86/268,494, JP 85/268,495, and JP 86/284,489.

The coating layer may also contain other additives, such as curing agents, preservatives, organic or inorganic fine particles, dispersing agents, antistatic agents, defoaming agents, viscosity-controlling agents, these and other ingredients having been described more fully in EP 133,011, EP 133,012, EP 111,004, and EP 279,467.

Any material can be used as the support for the dye-donor element provided it is dimensionally stable and capable of withstanding the temperatures involved, up to 400° C. over a period of up to 20 msec, and is yet thin enough to transmit heat applied on one side through to the dye on the other side to effect transfer to the receiver sheet within such short periods, typically from 1 to 10 msec. Such materials include polyesters such as polyethylene terephthalate, polyamides, polyacrylates, polycarbonates, cellulose esters, fluorinated polymers, polyethers, polyacetals, polyolefins, polyimides, glassine paper and condenser paper. Preference is given to a support comprising polyethylene terephthalate. In general, the support has a thickness of 2 to 30 μm. The support may also be coated with an adhesive of subbing layer, if desired.

The dye layer of the dye-donor element can be coated on the support or printed thereon by a printing technique such as a gravure process.

A dye-barrier layer comprising a hydrophilic polymer may also be employed between the support and the dye layer of the dye-donor element to enhance the dye transfer densities by preventing wrong-way transfer of dye backwards to the support. The dye barrier layer may contain any hydrophilic material that is useful for the intended purpose. In general, good results have been obtained with gelatin, polyacrylamide, polyisopropyl acrylamide, butyl methacrylate-grafted gelatin, ethyl methacrylate-grafted gelatin, ethyl acrylate-grafted gelatin, cellulose monoacetate, methylcellulose, polyvinyl alcohol, polyethyleneimine, polyacrylic acid, a mixture of polyvinyl alcohol and polyvinyl acetate, a mixture of polyvinyl alcohol and polyacrylic acid, or a mixture of cellulose monoacetate and polyacrylic acid. Suitable dye barrier layers have been described in e.g. EP 227,091 and EP 228,065. Certain hydrophilic polymers e.g. those described in EP 227,091 also have an adequate adhesion to the support and the dye layer, so that the need for a separate adhesive or subbing layer is avoided. These particular hydrophilic polymers used in a single layer in the dye-donor element thus perform a dual function, hence are referred to as dye-barrier/subbing layers.

Preferably the reverse side of the dye-donor element has been coated with a slipping layer to prevent the printing head from sticking to the dye-donor element. Such a slipping layer would comprise a lubricating material such as a surface-active agent, a liquid lubricant, a solid lubricant or mixtures thereof, with or without a polymeric binder. The surface-active agents may be any agents known in the art such as carboxylates, sulfonates, phosphates, aliphatic amine salts, aliphatic quaternary ammonium salts, polyoxyethylene alkyl ethers, polyethylene glycol fatty acid esters, fluoroalkyl C₂ -C₂₀ aliphatic acids. Examples of liquid lubricants include silicone oils, synthetic oils, saturated hydrocarbons, and glycols. Examples of solid lubricants include various higher alcohols such as stearyl alcohol, fatty acids and fatty acid exters. Suitable slipping layers have been described in e.g. EP 138,483, EP 227,090, U.S. Pat. No. 4,567,113, U.S. Pat. No. 4,572,860, U.S. Pat. No. 4,717,711. Preferably the slipping layer comprises a styrene-acrylonitrile copolymer or a styrene-acrylonitrile-butadiene copolymer or a mixture thereof or a polycarbonate as described in European patent application no. 91202071.6, as binder and a polysiloxane-polyether copolymer or polytetrafluoroethylene or a mixture thereof as lubicrant in an amount of 0.1 to 10% by weight of the binder or binder mixture.

The support for the receiver sheet that is used with the dye-donor element may be a transparent film of e.g. polyethylene terephthalate, a polyether sulfone, a polyimide, a cellulose ester or a polyvinyl alcohol-co-acetal. The support may also be a reflective one such as a baryta-coated paper, polyethylene-coated paper or white polyester i.e. white-pigmented polyester. Blue-coloured polyethylene terephthalate film can also be used as support.

To avoid poor adsorption of the transferred dye to the support of the receiver sheet this support must be coated with a special layer called dye-image-receiving layer, into which the dye can diffuse more readily. The dye-image-receiving layer may comprise e.g. a polycarbonate, a polyurethane, a polyester, a polyamide, polyvinyl chloride, polystyrene-co-arcylonitrile, polycaprolactone, or mixtures thereof. The dye-image receiving layer may also comprise a heat-cured product of poly(vinyl chloride/co-vinyl acetate/co-vinyl alcohol) and polyisocyanate. Suitable dye-image-receiving layers have been described in e.g. EP 133,011, EP 133,012, EP 144,247, EP 227,094, and EP 228,066.

In order to improve the light-fastness and other stabilities of recorded images UV-absorbers, singlet oxygen quenchers such as HALS-compounds (Hindered Amine Light Stabilizers) and/or antioxidants can be incorporated into the dye-image-receiving layer.

The dye layer of the dye-donor element or the dye-image-receiving layer of the receiver sheet may also contain a releasing agent that aids in separating the dye-donor element from the receiver sheet after transfer. The releasing agents can also be incorporated in a separate layer on at least part of the dye layer and/or of the dye-image-receiving layer. Suitable releasing agents are solid waxes, fluorine- or phosphate-containing surface-active agents and silicone oils. Suitable releasing agents have been described in e.g. EP 133,012, JP 85/19,138, and EP 227,092.

The dye-donor elements according to the invention are used to form a dye transfer image, which process comprises placing the dye layer of the dye-donor element in face-to-face relation with the dye-image-receiving layer of the receiver sheet and image-wise heating from the back of the dye-donor element. The transfer of the dye is accomplished by heating for about several milliseconds at a temperature of 400° C.

When the process is performed for but one single colour, a monochromic dye transfer image is obtained. A multicolour image can be obtained by using a dye-donor element containing three or more primary colour dyes and sequentially performing the process steps described above for each colour. The above sandwich of dye-donor element and receiver sheet is formed on three occasions during the time when heat is applied by the thermal printing head. After the first dye has been transferred, the elements are peeled apart. A second dye-donor element (or another area of the dye-donor element with a different dye area) is then brought in register with the dye-receiving element and the process is repeated. The third colour and optionally further colours are obtained in the same manner.

In addition to thermal heads, laser light, infrared flash, or heated pens can be used as the heat source for supplying heat energy. Thermal printing heads that can be used to transfer dye from the dye-donor elements of the present invention to a receiver sheet are commercially available. In case laser light is used, the dye layer or another layer of the dye element has to contain a compound that absorbs the light emitted by the laser and converts it into heat e.g. carbon black.

Alternatively, the support of the dye-donor element may be an electrically resistive ribbon consisting of e.g. a multilayer structure of a carbon-loaded polycarbonate coated with a thin aluminium film. Current is injected into the resistive ribbon by electrically addressing a printing head electrode resulting in highly localized heating of the ribbon beneath the relevant electrode. The fact that in this case the heat is generated directly in the resistive ribbon and that it is thus the ribbon that gets hot leads to an inherent advantage inprinting speed using the resistive ribbon/electrode head technology as compared to the thermal head technology, according to which the various elements of the thermal head get hot and must cool down before the head can move to the next printing position.

The following examples illustrate the invention in more detail without, however, limiting the scope thereof.

EXAMPLE 1

The absorption maxima (λ_(max)) of the dyes identified below were determined. The results are listed in Table 2.

                  TABLE 2                                                          ______________________________________                                                Dye   λ.sub.max (nm)                                             ______________________________________                                                C1 (a)                                                                               615                                                                      C2 (b)                                                                               565                                                                      C3 (c)                                                                               565                                                               ______________________________________                                          (a) determined in dichloromethane/methanol (2:8)                               (b) determined in dichloromethane/methanol (1:1)                               (b) determined in dichloromethane/methanol/dimethylformamide (1:8:1)     

EXAMPLE 2

Receiver sheets were prepared by coating a subbed polyethylene terephthalate film having a thickness of 175 μm with a dye-image-receiving layer from a solution in ethyl methyl ketone of 3.6 g/m² of poly(vinyl chloride/co-vinyl acetate/co-vinyl alcohol) (Vinylite VAGD supplied by Union Carbide), 0.336 g/m² of diisocyanate (Desmodur N3300 supplied by Bayer AG), and 0.2 g/m² of hydroxy-modified polydimethylsiloxane (Tegomer H SI 2111 supplied by Goldschmidt).

Dye-donor elements for use according to thermal dye sublimation transfer were prepared as follows.

A solution in methyl ethyl ketone of 0.5% by weight of dye C1 and 0,5% by weight of poly(styrene-co-acrylonitrile) (PSA) (Luran 388S, supplied by BASF Germany) was prepared.

A dye layer having a wet thickness of 100 μm was coated from this solution on a polyethylene terephthalate film support having a thickness of 6 μm and carrying a conventional subbing layer. The resulting dyelayer was dried by evaporation of the solvent.

The opposite side of the film support was coated with a subbing layer of a copolyester comprising ethylene glycol, adipic acid, neopentyl glycol, terephthalic acid, isophthalic acid, and glycerol.

The resulting subbing layer was covered with a solution in methyl ethyl ketone of 0.5 g/m² of a polycarbonate having the following structural formula to form a heat-resistant layer: ##STR14## wherein x=55 mol % and y=45 mol %.

Finally, a top layer of polyether-modified polydimethylsiloxane (Tegoglide 410, Goldschmidt) was coated from a solution in isopropanol on the resulting heat-resistant polycarbonate layer.

The dye-donor element was printed in combination with a receiver sheet in a Mitsubishi colour video printer CP100E.

The receiver sheet was separated from the dye-donor element and the colour density value of the recorded image was measured in reflex by means of a Macbeth TR 924 densitometer in the red, green, and blue regions in Status A mode.

The obtained values were as follows:

D_(max) =60; behind red filter 60, green filter 28 and blue filter 9. It can thus be seen that dye C1 exhibits a rather broad spectrum which makes the dye suitable for making black and white images. 

We claim:
 1. A dyed receiving element comprising a support having thereon a dye image receiving layer and a releasing agent selected from the group consisting of solid waxes, fluorine-containing surface-active agents, phosphate-containing surface-active agents and silicone oils, wherein said releasing agent is contained in said dye image receiving layer or in a separate layer on at least part of said dye image receiving a layer, said dyed receiving element further comprising at least one heterocyclic hydrazono dye in an image-wise distribution, wherein said at least one heterocyclic hydrazono dye corresponds to the following general formula (I): ##STR15## wherein: Z represents the atoms necessary to complete a heterocyclic ring system, or a substituted heterocyclic ring system, including a heterocyclic ring system carrying a fused-on aliphatic or aromatic ring system,Y represents the atoms necessary to complete a heterocyclic coupler system or substituted heterocyclic coupler system, R² represents an alkyl group, a substituted alkyl group, a cycloalkyl group, a substituted cycloalkyl group, an aryl group, or a substituted aryl group, m is 0 or 1, and each of Q₁ and Q₂ independently represents hydrogen or a substituent. 